The present invention pertains to the design of a permanent or "hard" magnet in a magneto-resistive sensor. More particularly, the present invention pertains to a hard magnet which has increased height or thickness and the resultant decrease in resistance and increased magnetic strength while maintaining the directional attributes of a thinner hard magnet structure. The objective of the invention is to optimize the design and performance of the magneto-resistive (MR) sensor.
MR sensors or heads are used to read magnetically encoded information from a magnetic medium by detecting magnetic signal stored in the magnetic medium, or, more precisely, by detecting changes in magnetic flux while moving immediately above the magnetic medium. The MR sensor has an "active area" which includes a layer or strip of magneto-resistive material, i.e, a material with an electrical resistance which varies responsive to changes in magnetic flux. During the operation of an MR sensor, a sense current is passed through the active area, with the resistance of the active area causing a voltage drop. The magnitude of the voltage drop is at least partially a function of the resistance of the MR layer in the active area.
The MR sensor is suspended immediately adjacent a magnetic medium while the magnetic medium moves relative to the MR sensor. The magnetic medium has magnetic "transitions", i.e., changes in direction of magnetic alignment, which were previously recorded by the write head. As the direction and magnitude of the magnetic flux running through the MR sensor varies, the voltage drop across the active area also varies. The magnetic transitions are accordingly detected by monitoring the voltage across the active area of the MR element.
The change of resistance across the MR element per change in magnetic flux has been found to depend upon the square of the cosine of the angular relationship between the direction of magnetization (magnetization vector M) of the MR element and the direction of electrical current (current density vector J) flowing through the MR element. An MR sensor will provide its most linear output, that is, approximately equal and opposite changes in output for corresponding equal and opposite changes in magnetic flux, when these two vectors form an angle of 45.degree.. A generally linear output from the MR sensor is necessary to achieve optimum sensitivity and minimum readback signal distortion, to provide a signal which can be easily and accurately read.
Permalloy is an alloy of nickel and iron which is typically used as an MR material. When permalloy is formed into a long narrow strip such as commonly used in the active region of a MR sensor, it naturally tends to form a magnetization vector along its long axis. This alignment can be enhanced by a field induced anisotropy formed during the deposition of the permalloy element. The current density vector is also naturally orientated along the long axis of the MR element, in the same direction as the natural alignment of the magnetization vector, leading to an angle between these two vectors approaching 0.degree..
Various techniques have been used to increase the angle between the magnetization vector and the current density vector to more nearly approach 45.degree.. For instance, a soft adjacent layer (SAL) may be used adjacent the MR element. In conjunction with the SAL or otherwise, permanent or hard magnets may be placed adjacent the MR element.
The hard magnets are commonly formed of a Cobalt based alloy with a sufficiently high coercivity, which is sufficiently magnetized and perhaps shielded so the magnetic fields of the media and/or the write head do not effect the magnetism of the hard magnets. That is, the hard magnets once constructed should be permanently magnetized in the field of use. To perform effectively, the hard magnets should have a high coercivity, high MrT and have high in-plane squareness on the magnetization curve.
Located adjacent the MR element, the hard magnet structure not only affects the magnetism in the active area of the MR layer, but also is within the flow path of current to and from the active area of the MR layer. As in the MR layer, both the magnetic and the electrical properties of the hard magnet structure are important to maximize signal readability. The magnetic field of the hard magnets should be large enough to keep the active area of the MR layer in a single domain state to suppress the "Barkhausen" noise, yet small enough so as not to reduce the sensitivity of the MR sensor to magnetic flux changes.
MR elements can "fracture" into multiple magnetic domains when they are exposed to an external magnetic field, creating so called "Barkhausen noise" in the resulting output from the MR sensor. To maximize the MR sensor's output stability, it is desirable to maintain the MR element in a single domain state. The structure of the hard magnets play a vital role in stabilization of the MR element in a single domain state.